Is there a way to force nginx to treat variable as upstream name, not URL?

I have a number of upstreams that I select from based on a set of headers, but am finding after the addition of https://github.com/GUI/nginx-upstream-dynamic-servers that my $destination variable is being interpreted as a URL and not an upstream. Here's a snippet:

http {
    upstream LegacyService {
        server my.server.location.com:443 max_fails=0 resolve;
    }


    upstream NewService {
        server myNew.server.location.com:443 weight=100 max_fails=0 resolve;

    }

    ... 
    map $http_some_header $destination {
        default LegacyService;
        "~marker" NewService;
    }
    
    server {
        listen localhost:8080;
        
        # lots of the usual setup
        
        location / {
            proxy_pass https://$destination;
        }
    }
}

This configuration gets me a bunch of errors as now my destinations are interpreted as URLs instead of the names of upstreams:

2021/08/09 11:05:56 [error] 15326#0: *5761 no live upstreams while connecting to upstream, client: 10.1.1.5, RequestID: 6ef3b58fc95e07b083e6186df62ba15d, server: my.server.com, request: "POST / HTTP/1.1", upstream: "https://LegacyService/", host: "my.server.com"

I'd like to understand why this change is happening and see if there's anything that can be done. Am I misreading the error message?